This invention relates to a light-emitting semiconductor device, or light-emitting diode (LED) according to more common parlance, and more particularly to such devices employing gallium-containing compound semiconductors. The invention also concerns a method of making such light-emitting semiconductor devices.
The LED has been known which has a light-generating semiconductor region grown on a substrate of electrically conducting material such as gallium arsenide. Typically, the light-generating semiconductor region has an active layer sandwiched between an n-type cladding or lower confining layer, which overlies the substrate, and a p-type cladding or upper confining layer. An anode is mounted centrally atop the upper confining layer whereas a cathode underlies the substrate. The light generated at the active layer partly traverses directly through the upper confining layer and issues from that part of the surface of the semiconductor region which is left uncovered by the anode. The rest of the light is radiated toward the substrate via the lower confining layer. How to reflect this light most effectively back toward the light-emitting surface of the semiconductor region is of critical importance for the highest possible efficiency of the LED.
One conventional solution to this problem is a reflective film known as the Bragg reflector interposed between the substrate and the light-generating semiconductor region. The Bragg reflector is easy to fabricate by epitaxial growth, the method adopted for subsequent formation of the semiconductor region. Offsetting this advantage is the lack of sufficient reflectivity with respect to the light having a wide spectrum of wavelengths.
Another prior art method calls for the removal of the gallium arsenide substrate following the epitaxial growth of the semiconductor region thereon. A transparent baseplate is then bonded to the semiconductor region in place of the substrate that has been removed, by way of a mechanical support for the LED. Then a reflective electrode is attached to the transparent baseplate. The reflective electrode serves not only as electrode but to reflect the light back through the transparent baseplate toward the light-emitting surface of the semiconductor region. This known remedy is objectionable for a relatively high forward voltage required between anode and cathode as a result of additional resistance at the interface between light-generating semiconductor region and transparent baseplate.
Japanese Unexamined Patent Publication No. 2002-217450, filed by the assignee of the instant application, represents an improvement over the more conventional devices listed above. It teaches the creation of isolated ohmic contact regions of gold-germanium-gallium alloy on the underside of the light-generating semiconductor region. These ohmic contact regions, as well as the surface of the semiconductor region left uncovered thereby, are covered by a reflective layer of aluminum or other metal. An electroconductive baseplate is bonded to the underside of the reflective layer. Making good ohmic contact with the light-generating semiconductor region of, say, aluminum gallium indium phosphide, the ohmic contact regions of gold-germanium-gallium alloy serve for reduction of the forward voltage of the LED.
The last cited prior art LED proved to possess its own weaknesses, however. The gold-germanium-gallium ohmic contact regions were rather inconveniently absorptive of light by reasons of their germanium content and thickness in particular. The total reflectivity of the ohmic contact regions and reflective layer was therefore as low as 30 percent or thereabouts, making it difficult for the LED to gain sufficiently high efficiency. Another shortcoming concerned the morphology of the gold-germanium-gallium ohmic contact regions: Their surfaces were so uneven that difficulties were experienced in bonding the electroconductive baseplate thereto via the reflective layer.